51% attack
An attack where one party controls most of the network's mining power or stake and can rewrite recent history.
In simple terms
With the majority of the work, an attacker can rebuild blocks faster than the honest network and reverse recent transactions — but cannot steal coins outright or invent new ones.
Analogy
A vote count where one party staffs most of the counting tables: they can recount to a preferred result, but cannot add voters who never showed up.
Going deeper
A majority can double-spend and censor, not counterfeit: signatures still protect other people's coins, and old confirmations stay buried under re-mined work. Real attacks have hit small PoW chains where rented hashrate exceeded the honest network.
Common mistake
Confusing "can reverse recent confirmations" with "can break the whole system" — deep history and foreign signatures stay safe.